1. Field of the Invention
The present invention relates to image processing and more particularly to a method for processing an image meant to be displayed with a reduced number of colors.
2. Description of the Related Art
Displaying images on a screen that has a restricted number of colors requires specific processing in order to avoid defects that are particularly annoying if no precaution is taken.
Indeed, in the absence of any corrective processing, when displaying an image coded, for example, into 16 million colors on a screen having a restricted number of colors (for example 4096 colors) one observes the appearance of moiré effects that do not form part of the original image. In video, flickering appears, moving areas that do not form part of the original video. In short, a number of localized visual defects generally known in the art as artifacts.
A number of techniques are known for reducing the occurrence of such artifacts. Generally, the known techniques are the product of a common approach, known as DITHERING. In all these techniques, small variations are made among the set of displayable colors (which is assumed to be restricted) so that these variations are assimilated by the eye and give the illusion of displaying a greater number of colors.
The first known technique—undoubtedly the simplest to implement—is based on the addition, for each displayed pixel, of a random noise to each color component. This technique makes it possible to somewhat reduce the moiré effect. On the other hand, the addition of random noise clearly damages the quality of the original image and generates grains in the resulting image as well as flickering in the case of a video.
In a second technique, known as ORDERED DITHERING, the image is processed before display by replacing areas containing non-displayable gradations by areas comprising displayable patterns, by analyzing a group of adjacent pixels. This technique is covered by the article: “Frequency analysis of ordered dither”, SPIE vol 1079, 1989, p. 361-373, Ulichney and Robert. Generally, although it improves processing compared to the preceding technique, this technique produces new patterns (or artifacts) that are easily recognizable by the human eye. Moreover, in the case of a video, this technique causes flickering, and even moving areas that do not form part of the original video.
A third known technique, and undoubtedly the most sophisticated one, is described in the reference document “An adaptive algorithm for spatial grayscale” proceedings of the society for information display, vol 17, Nb2, p. 75-77, 1976, Floyd and Steinberg. This technique proceeds by diffusion of the error on one pixel (between the original image and the displayed image) onto adjacent pixels. Implementing this technique requires great computing power and consequently turns out to be expensive. Moreover, although it makes it possible to remove moiré effects and flickering, one nevertheless notes the appearance of a very grainy aspect that finally proves cumbersome. It is especially true for a video as it transforms into swarming.
All the known techniques that have been mentioned are classically used to process fixed images. Clearly, video image sequences require specific processing to avoid the appearance of other objects or artifacts that might not be perceived if the known techniques were applied to fixed images.
Accordingly, there exists a need for overcoming the disadvantages of the prior art as discussed above.